World War I

...German assaults on neutrals’ rights at sea, and the cumulative effect of Allied propaganda and German provocations conjoined to end U.S. neutrality by 1917. On February 4, 1915, Germany declared the waters around the British Isles a war zone in which Allied ships would be sunk, without warning if necessary. While this procedure dispensed with traditional civilities like boarding, search and...

...economy with a campaign against its supply lines of merchant shipping. In 1915, however, with their surface commerce raiders eliminated from the conflict, they were forced to rely entirely on the submarine.

...the German submarines could do any decisive damage to Great Britain so long as their warfare was restricted in deference to the protests of the United States; and, after a tentative reopening of the submarine campaign on Feb. 4, 1916, the German naval authorities in March gave the U-boats permission to sink without warning all ships except passenger vessels. The German civilian statesmen,...

World War II

In both world wars, submarines were also a serious threat to merchant shipping. In World War II, German U-boats nearly severed the lifelines to Great Britain, U.S. submarines successfully isolated Japan by nearly wiping out its merchant fleet, and, in the Mediterranean, British and Axis submarines vied in attempts to cut their opponents’ communication with North Africa.